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What are 3 indications for brain SPECT imaging?
- 1. Blood flow studies
- 2. Neuronal function
- 3. CSF dynamics
what type of drugs are allowed in the brain and what type of drugs are blocked from the brain?
Fat soluble drugs are allowed in the brain and water soluble drugs are blocked from the brain
Does the drug have to be ionized or ionized to enter the brain, does it have to be free or bound to enter the brain, and does it have to be a hydrophilic or lipophilic?
The drug has to be free, un ionized, and fat soluble.
Where is pertechnetate found and what is it's adult dose?
- Salivary glands, gastric mucosa, thyroid, choroid plexus
- Slow plasma clearance
what are some non diffusible tracers not use for brain imaging anymore?
- Pentetate (DTPA)
- glucepate (GH)
- Rb82 chloride
These are not used anymore because of poor anatomic resolution
How are non diffusible tracers localizeD?
- Confined in a intravascular space
- Enter the brain at sites of BBB breakdown
- normal brain is cold and lesions are hot
How are the diffusible tracers is localised
- Enter brain by passive diffusion through intact BBB
- Trapping occurs: HMPAO and ECD are localized
- Normal brain is hot and lesions are cold
What are some diffusible tracers that look at brain function
- Exametazime (HMPAO)
- bicisate (ECD)
- N13 ammonia
If HMPAO is lipophilic how does it stay in the brain?
It is trapped in the brain by converting unionize to ionize by glutathione, which is an intracellular reducing agent (changes lipophilic to hydrophilic)
What are the 3 reasons for doing cerebral blood flow studies?
- 1. Brain death
- 2. Epilepsy
- 3. Cerebral vascular disease
- - ischemia
What are the 3 basic radiotracer requirements for assessing Regional cerebral blood flow (rCBF)?
- 1. has to cross the BBB
- 2. Brain retention long enough to acquire images
- 3. It has to remain localize
brain flow studies are best done by what diffusible tracers and why?
HMPAO and ECD,
because any activity reaching the brain will remain fixed in the brain which is a more sensitive study
Why is there no flow to the brain in brain death studies?
Because the brain swells and prevents blood from entering the brain (intracranial pressure due to edema)
What does the "hot nose" sign mean in HMPAO brain death study?
Shunting of blood flow to nasopharynx via external carotid arteries
Where does blood flow in the brain seen on SPECT images?
Blood flow to seizure focus increases when?
During ictus (seizure) and it's diminished to normal brain between seizures (intraictally)
How is epilepsy imaging performed?
- 1. EEG monitoring
- 2. HMPAO or ECD is used during ictus because of high first pass extraction across the BBB and remain fixed at the focus
- 3. Tracer best injected within first 20 seconds of seizure onset because 60 percent of patients show hyperperfusion if tracer is injected within 100 seconds of seizure termination (post ictus)
- 4.SPECT imaging following 15-30 mCi can begin as 30-60 min and 4hrs post seizure
Define ictal and postictal SPECT?
- Ictal: inject within 30 seconds
- Postictal: 30 seconds -5 minutes
Best to compare both because ictal or Postictal image can appear normal if read alone, but may show increased perfusion relative to hypoperfusion at the same time
What tracer is best used in interictal time
- hypoperfusion is present in all patients after 100 second following a seizure; 1.shows hypometabolism (glucose metabolism is reduced at the seizure focus compared to normal brain)
2. because of the extended time required for uptake by facilitated diffusion and intracellular phosphorylation to FDG phosphate
what are some causes of cerebral vascular disease?
- Diabetes mellitus (DM)
- carotid artery stenosis
- AV malformations
These can lead to TIA (transient ischemic attacks) or stroke (acute cerebral vascular ischemia)
What is cerebrovascular reserve (CVR)?
Autoregulatory response to improve blood flow by causing vasodilation as carbon dioxide accumulates do to a drop in perfusion pressure.
* non compliant regions with reduce CvR may exhibit normal blood flow at rest, but demonstrate reduce blood flow when challenge with a vasodilator
Describe the acetazolamide stress test
- 1. Give 1g acetazolamide IV over 2 minutes (elevates CO2 which dilates vessels)
- 2. After 20 minutes inject 30 mCi of HMPAO ( measures brain perfusion)
- 3. give HMPAO without acetazolamide
- 4. Compare rest and stress studies
*This study the detects whether an adequate reservoir exist ( if patient loses blood flow the reservoir provides bring with emergency blood)
What are the 2 metabolism tracers for brain imaging?
- -hydrophilic:facilitated diffusion
- Tl-201 Chloride
- -active transport
How does the uptake look of F18-FDG in epilepsy, tumor regrowth, and dementias?
- Epilepsy: ictus increase; interictus decreased uptake
- Tumor regrowth: increase uptake due to tumor metabolism; decreased uptake due to differentiated tumor necrosis
- Dementias: decreased uptake
Why is FDG and Florbetapir used in Alzheimer's patients?
**Amyvid is not definitive for AD**
- FDG: measures altered metabolic glucose distribution
- Florbetapir (Amyvid): demonstrates increased uptake into beta amyloid plaques and neurofibrillary tangles
What is the purpose of F18 Fluorodopa injection?
- -Measures dopamine synthesis
- -Enzymatic marker of dopa decarboxylase activity.
- -has decreased uptake in dopaminergic neurons as disease severity increases (Parkinson's)
What is the purpose of I-123 Ioflupane (DATscan)?
Distinguishes patients with essential tremor from patients with tremor associated with dopaminergic deficit related to Parkinson's Disease
- -Measures dopamine transporter function (DAT)
- -has decreased uptake in dopaminergic neurons as Parkinson's severity increases
What are 2 CSF agents?
In-111 DTPA and Tc99m DTPA
- -show abnormal CSF dynamics, shunts, and leaks
- -hydrophilic, remains confined in CSF
What is cisternography?
- Evaluation of CSF dynamics
- -use In-111 DTPA
- -NORMAL DISTRIBUTION:excluded from ventricles
- -HYDROCEPHALUS: enters ventricles
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